Application of Thermal Methods to Analyze the Properties of Coffee Silverskin and Oil Extracted from the Studied Roasting By-Pro

applied sciences

Article Application of Thermal Methods to Analyze the Properties of Coﬀee Silverskin and Oil Extracted from the Studied Roasting By-Product

Agata Górska 1,*, Rita Brzezi ´nska 1, Magdalena Wirkowska-Wojdyła 1, Joanna Bry´s 1 , Ewa Domian 2 and Ewa Ostrowska-Lig˛eza 1

1 Department of Chemistry, Institute of Food Sciences, Warsaw University of Life Sciences, 166 Nowoursynowska Street, 02-787 Warsaw, Poland; [email protected] (R.B.); [email protected] (M.W.-W.); [email protected] (J.B.); [email protected] (E.O.-L.) 2 Department of Food Engineering and Process Management, Institute of Food Sciences, Warsaw University of Life Sciences, 166 Nowoursynowska Street, 02-787 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +4822-5937611

Received: 14 November 2020; Accepted: 7 December 2020; Published: 9 December 2020

Featured Application: Coffee silverskin is a valuable by-product of the coffee roasting process with a wide range of potential applications. Fat extracted from the studied material turned out to be a source of essential fatty acids with the recommended n-6 to n-3 ratio and could be successfully recovered and reused in other fields of industry, including, especially, the food and cosmetics industries, which fits perfectly with current trends in circular ecology. Thermal analysis can be applied to obtain quick and reliable information about the composition, phase transition and transformation kinetics of the tested sample.

Abstract: The aim of the study was to characterize the thermal properties of coffee silverskin and fat extracted from the material by using differential scanning calorimetry, modulated differential scanning calorimetry and thermogravimetry/derivative thermogravimetry. Additionally, the thermokinetic parameters, oxidative stability and fatty acid composition of the extracted oil were defined. Thermal decomposition of the studied coffee roasting by-product under oxygen occurred in three defined stages. The most significant changes in weight were observed in the region of 200–500 ◦C and correspond to polysaccharide decomposition. These results are in agreement with the data obtained from the differential scanning calorimetry curve. On the curve course of silverskin, two main exothermic peaks can be observed with a maximum at 265 and 340 ◦C. These exothermic events represent the transitions of hemicellulose and cellulose. Fat extracted from silverskin turned out to be a source of polyunsaturated fatty acids with the recommended n-6 to n-3 ratio reaching the value 4:1. The studied fat was characterized by low oxidative stability. Considering the obtained results, it can be stated that thermal analysis can provide fast and reliable data concerning the composition and properties of coffee silverskin and coffee silverskin oil.

Keywords: thermal analysis; differential scanning calorimetry; thermogravimetry; coffee silverskin; coffee silverskin oil

1. Introduction Coffee is one of the most consumed beverages in the world and is produced in great amounts [1]. Among the many steps in coffee production, the roasting process seems to be of great importance, as under the influence of high temperature, many physicochemical changes occur. During the roasting

Appl. Sci. 2020, 10, 8790; doi:10.3390/app10248790 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8790 2 of 15 process, an outer layer of green coffee beans, known as coffee silverskin, is removed [2]. Generally, green coffee beans with attached tegument (silverskin) are exported to consuming countries, where the coffee beans undergo the roasting process [3]. The amount of this waste depends on the type of green beans and on the conditions of processing used, but mostly, coffee silverskin represents about 4.2% (w/w) of beans [4]. According to the International Coffee Organization (ICO) statistics [5], total production of coffee by all exporting countries is estimated at 10 million tonnes. Considering the datum presented above, the coffee industry might generated approximately 420 thousand tonnes of the studied by-product. However, this agro-industrial residue currently presents little commercial value and is mostly disposed of as industrial waste. Taking the aforementioned reasons into account, it is imperative to identify the properties and potential applications of coffee silverskin. The reuse of a waste product, such as silverskin, would fit with current trends in circular ecology [6]. In recent years, the problem of development in improving the efficiency of raw material processing has been widely discussed. There is considerably little research conducted on the topic of coffee by-products’ properties. Publications mainly deal with antioxidant and prebiotics characteristics [7–10]. Results show that coffee silverskin can be reused in many industrial fields due to its functional and nutritional properties [11–13]. In the case of coffee by-products, there is still a lack of information concerning their thermal behavior, which is of great importance for defining the potential applications of this waste product. Among the thermal techniques that enable getting valuable information about tested materials are differential scanning calorimetry (DSC), modulated differential scanning calorimetry (MDSC) and thermogravimetry/derivative thermogravimetry (TG/DTG). DSC is a technique which is particularly useful in terms of food systems which undergo heating or cooling during processing. The information obtained from DSC curves can be helpful in understanding thermal transitions under certain treatment of the material [14–16]. Additional details about the studied samples can be obtained by using thermogravimetry analysis/derivative thermogravimetry. In this technique, mass loss of the material under controlled linearly varying or isothermal temperatures over time under a given atmosphere is determined. It can provide information about physical phenomena, including phase transitions and chemical changes, such as thermal decomposition [17]. Thermal techniques are widely used to obtain information about foods systems [18]. DSC is the most universal thermal analytical technique applied to detect phase and state transitions of food products containing mainly proteins and polysaccharides. MDSC is especially useful in determining glass transition temperature, which is important in determining material stability [15]. Górska et al. [19] studied the glass transition temperature of newly designed β-lactoglobulin—retinyl palmitate—disaccharide powdered systems. Based on the obtained results, the impact of the composition of the studied samples on the glass transition temperature was shown. In the case of trehalose incorporation into products, the temperature was higher than in samples containing lactose. In food systems, a higher glass transition temperature can be assumed to improve protection and stability of encapsulated substances. Calorimetry can also provide valuable information about thermal properties of proteins and, what is helpful in understanding the stability of proteins, the forces that maintain their folded structures and interactions with other macromolecules as a function of temperature [20]. Thermodynamic analysis of DSC was also applied to investigate thermal denaturation and aggregation of food proteins. It is possible to estimate the apparent denaturation enthalpy after heating protein solutions at a chosen temperature and time range. Wang et al. [21] studied the kinetics of the aggregation of α-lactoalbumin at 90 ◦C and found that kinetic curve of the process can be described by the first-order reaction equation with the constant rate of about 4 1 10− s− . DSC is also a valuable technique to study the influence of chemical and physical treatments used during food preservation on inactivation of bacteria. Calorimetric data provided information that can be useful in optimizing the processing conditions of food preservation, in obtaining quantitative data about bacteria inactivation process, such as thermal energy required for bacteria inactivation and kinetic parameters of inactivation [22]. Lee and Kaletunç [23] studied process of Escherichia coli inactivation. E.coli samples were preheated in the DSC to set the temperature, cooled in liquid nitrogen and rescanned to 140 ◦C. On the DSC scan, the thermally induced transitions’ connection with the Appl. Sci. 2020, 10, 8790 3 of 15 surviving of bacterial cells could be observed. Based on the DSC curve course, apparent enthalpies of the process were calculated. Thermal analysis is also widely used in fat analysis. It gives the possibility to obtain crystallization and melting profiles obtained on cooling and heating of the sample; to monitor polymorphic forms of fats; to measure the heat of transitions; to define solid fat content in the sample; to monitor oxidation of lipids [18]. The problem of cocoa butter is often presented in the literature, as the quality of chocolate depends on the polymorphic forms of the fat. Based on thermal analysis, it can be shown that cocoa butter can be described in terms of six different polymorphic forms, with the commercially desired form V, which is not the most stable one, and form VI, which is more stable and undesirable, as responsible for fat bloom [24]. DSC studies of anhydrous milk showed that it crystallizes and melts in several steps [25]. Based on the DSC curve course, the presence of three endothermic peaks, corresponding to low melting, medium melting and high melting fractures, was observed, which differ in triacylglycerol structure. Thermal properties of fat in complex products are also the topic of scientific investigations. DSC was applied to a study of the thermal properties of fat in cheese. Lopez et al. [26] studied the crystalline structures in Emmental cheese at 4 ◦C and its melting characteristics during heating. They showed that the phase transition that occurred upon cooling on the DSC curve is related to fat globules’ destabilization. Calorimetry is also thought to be a quick and reliable technique to determine the oxidative stability of fats. In isothermal processes, the oxidation induction time is defined, which corresponds to exothermic reaction between lipid and oxygen. Induction time can be used as primary parameter for the assessment of the resistance of tested fat to oxidative deterioration. In non-isothermal experiments, kinetic parameters can be calculated, such as activation energy of oxidation process. Brynda-Kopytowska et al. [27] studied the oxidative stability of fat extracted from newly designed spray-dried pea-protein fat preparations formulated with different types of carbohydrate components. Authors observed that after 6 months of storage, fat isolated from spray-dried products formulated with inulin and trehalose containing the mixture of palm and rapeseed oils were the most stable. Wirkowska-Wojdyła et al. [28] investigated the oxidative stability of model meat batters as affected by interesterified fat. Fats extracted from meat batters were characterized by lower resistance to oxidation than fats used in the interesterification process. It can be due to a higher content of free fatty acids and polar fraction in final products which can reduce the stability of fat against oxidation. Ciemniewska et al. [29] applied pressure differential scanning calorimetry to assess the oxidative stability of fat isolated from roasted hazelnuts. The results have shown a slight elevation in the oxidative stability as the temperature and time of roasting process increased. There are many reports confirming that DSC can provide valuable data for vegetable oil oxidation, which correlate with Rancimat and electron spin resonance spectroscopy results [30,31]. Taking the above into consideration, the main aim of the study was to analyze the thermal properties of coffee silverskin obtained as a mix of the two most common species of coffee (Coffea arabica and Coffea canephora). Additionally, fat extracted from the studied material was characterized by oxidative stability and thermokinetic parameters. Thermal characteristics of studied samples was determined by using the following techniques: differential scanning calorimetry, modulated differential scanning calorimetry and thermogravimetry/derivative thermogravimetry. The research was completed by defining fatty acid composition based on chromatographic analysis of fatty acid methyl esters.

2. Materials and Methods

2.1. Study of Coffee Silverskin Coffee silverskin samples of the two most common species of coffee (Coffea arabica and Coffea canephora) of various geographical origins were studied. The sampling plan for the silverskin was developed on the basis of the procedure for taking samples from unpackaged batches according to PN-ISO 3534-2:1994 standard [32]. First, six times every 3 days, original silverskin samples were collected from coffee roasteries. The collected primary samples of silverskin were mixed to obtain a general sample. Representative laboratory samples used for testing were collected from the general Appl. Sci. 2020, 10, 8790 4 of 15 sample. Laboratory samples of silverskin were homogenized in a laboratory mill to form a powder and stored in polyethylene bags with a slider without light at room temperature.

2.1.1. DSC Study of Coffee Silverskin The TA DSC Q200 differential scanning calorimeter (TA Instruments, New Castle, DE, USA) was used for DSC measurements of the samples. The cell was purged with 50 mL/min dry nitrogen and calibrated using standard pure indium. An empty sealed aluminum pan was used as a reference. Samples (12–13 mg) were hermetically sealed in aluminum pans and heated from 50 to 450 C at a − ◦ heating rate of 5 ◦C/min. As a result, experiment curves of heat flow (W/g) versus temperature were obtained. The thermal parameters were defined with the use of the Universal V4.5A (TA Instruments, New Castle, DE, USA) program [14].

2.1.2. MDSC Study of Coﬀee Silverskin Modulated DSC analyses were performed in order to determine the glass transition temperature of the samples. Purging of the cell was performed with 50 mL/min of nitrogen. The apparatus was calibrated using pure indium standard. As the reference in each test, an empty sealed aluminum pan was used. Samples of 11–15 mg in weight were sealed hermetically in 30-µL aluminum pans. Subsequently, samples were cooled from room temperature to 50 C at a cooling rate of 5 C per min − ◦ ◦ and then equilibrated for 5 min. After the equilibration period, samples were scanned from 50 to − 200 C at a constant heating rate of 2 C per min with an amplitude of 1 C and a 60-s period of ◦ ◦ ± ◦ modulation. Glass transition was reported with parameters indicating its midpoint of vertical shift in the reversing transition curve. TA Instruments Universal analysis software was used to analyze the glass transition temperature. The measurement were done in three replicates for each sample [15].

2.1.3. TG/DTG Study of Coffee Silverskin The samples were also tested using a Discovery TGA thermogravimetric analyzer (TA Instruments, New Castle, DE, USA). Measurements were performed under nitrogen and oxygen at a flow rate of 25 mL/min at a temperature range of 50–700 ◦C with a heating rate of 10 ◦C/min. Approximately 7–8 mg of the sample was placed on the thermobalance in a platinum container. TG curves were obtained for temperature dependence on mass loss, and first-derivative data (DTG) were calculated [17].

2.2. Study of Fat Extracted from Coﬀee Silverskin

2.2.1. Extraction of Fat from Coffee Silverskin Fat extraction from silverskin samples was conducted according to the procedure described by Reder et al. [33]. The samples were ground and treated with hexane. After 30 min of shaking, the mixture was filtered and dried with magnesium sulfate and the solvent was evaporated from the filtrate. Additionally, the sample was dried under nitrogen atmosphere to remove residual hexane.

2.2.2. Pressure Differential Scanning Calorimetry (PDSC) Study of Coffee Silverskin Oil PDSC experiments were carried out using a DSC Q20 TA Instrument (TA Instruments, New Castle, DE, USA). Fat samples of 3–4 mg were placed in the sample chamber under oxygen atmosphere with an initial pressure of 1400 kPa. The maximum PDSC oxidation time (induction time) was determined based on the maximum rate of oxidation (maximum rate of heat flow) with a 50 mL/min oxygen flow rate. The isothermal temperature for individual experiments was 100, 110 and 120 ◦C. Obtained diagrams were analyzed using TA Universal Analysis 2000 software. The maximum PDSC oxidation time was determined based on the maximum rate of oxidation as a peak’s point of maximum deviation from a linear baseline [34]. The non-isothermal version of PDSC was also used. The samples were heated from 30 to 250 ◦C at the rates of 2.5, 4.0, 5.0, 7.5, 10.0 and 12.5 ◦C per minute. The experiments were performed in an Appl. Sci. 2020, 10, 8790 5 of 15 atmosphere of oxygen with an initial pressure of 60–70 kPa and the gas flowing at a rate of 50 mL/min. For each programmed heating rate (β, ◦C/min), an analysis was done at least three times. The onset oxidation temperature (ton, ◦C) was determined as the intersection of the extrapolated baseline and the tangent line (leading edge) of the recorded exotherm. The kinetic parameters of the oxidation process (activation energy, pre-exponential factor and reaction rate constants) were calculated [34].

2.2.3. TG/DTG Study of Coﬀee Silverskin Oil Samples of fats were characterized with the use of a Discovery TGA thermogravimetric analyzer. Measurements were performed using methodology described above for silverskin analysis [17].

2.2.4. GC Measurements of Coffee Silverskin Oil Gas chromatography (GC) was used to determine the composition of fatty acids in silverskin fat samples. The fatty acids were converted into volatile methyl esters using a methanolic KOH solution according to PN-EN ISO 5509:2001 [35] and applied to the analytical column. The YL6100 GC gas chromatograph apparatus (Young Lin Bldg., Anyang, Hogye-dong, Korea) with a flame ionization detector and a 30-m long BPX 70 capillary column (SGE Analytical Science, Milton Keynes, UK) with an inner diameter of 0.22 mm and a film thickness of 0.25 µm was used. The oven temperature was programmed as follows: 60 ◦C for 5 min, then it was increased by 10 ◦C/min to 180 ◦C and from 180 to 230 ◦C at 3◦C/min. The temperature was kept at 230 ◦C for another 15 min. The temperature of the split injector was 225 ◦C, with a split ratio of 1:100. The detector temperature was kept at 250 ◦C. Nitrogen was applied as a carrier gas, with flow at the rate of 1 mL/min. Fatty acid identification was done based on the retention time compared to the standard. For the quantitative analysis of fatty acids, the area of the peak in the chromatogram was determined and the percentage of a given fatty acid was calculated.

2.3. Statistical Analysis Each measurement was taken in triplicate. The data are reported as the means standard deviation. ± One-way ANOVA was conducted using Statgraphics Plus for Windows program, version 4.1 (Statistical Graphics Corporation, Warrenton, VA, USA). Diﬀerences are considered to be signiﬁcant at a p-value of 0.05, according to Tukey’s multiple range test.

3. Results and Discussion

3.1. Study of Coﬀee Silverskin

3.1.1. DSC Study of Coffee Silverskin To characterize the thermal behavior of the samples of silverskin, differential scanning calorimetry (DSC) and thermogravimetry (TG) under nitrogen and oxygen flow were performed. The DSC curve presented in Figure1 shows the thermal transitions of the silverskin sample which occur between 60 C and 450 C at a heating rate of 5 C/min under a nitrogen atmosphere. − ◦ ◦ ◦ Appl. Sci. 2020, 10, 8790 6 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 16

FigureFigure 1. 1. DifferentialDiﬀerential scanning scanning calorimetry calorimetry (DSC) (DSC) curve curve of of coffee coﬀee silverskin silverskin sample. sample.

ConsideringConsidering the DSCDSC curve curve course, course, three three endothermic endothermic and twoand exothermic two exothermic events canevents be observed. can be observed.An early eventAn early observed event at observed 70 ◦C is associatedat 70 °C is with associated water evaporationwith water fromevaporation the sample. from From the sample. the DSC Fromprofile, the two DSC other profile, transitions two other that occurred transitions at higher that occurred temperatures at higher can be temperatures observed. These can thermalbe observed. events Thesepresent thermal at 125 andevents 240 present◦C are related at 125 to and the 240 presence °C are of related disaccharides to the inpresence the studied of disaccharides material. It is knownin the studiedthat coff material.ee silverskin It is is aknown material that containing coffee silverskin carbohydrates, is a material with polysaccharides containing carbohydrates, as the most abundant with polysaccharidescomponents [13 ].as This the comostffee by-productabundant components is a rich source [13]. of totalThis fiber,coffee including by‐product soluble is a dietary rich source fiber and of totalinsoluble fiber, dietaryincluding fiber soluble with highdietary amounts fiber and of cellulose,insoluble hemicellulosedietary fiber with and high lignin. amounts According of cellulose, to results hemicellulosepublished by Ballesterosand lignin. et According al. [4], cellulose to results and hemicellulosepublished by areBallesteros present inet silverskin al. [4], cellulose in significant and hemicelluloseamounts of 40.45% are present (w/w) ofin thesilverskin composition in significant of a dry weightamounts basis. of 40.45% Another (w fraction/w) of the that composition is represented of aby dry 28.58% weight (w basis./w) is Another lignin. On fraction the DSC that curve is represented course of by the 28.58% studied (w silverskin,/w) is lignin. two On main the DSC exothermic curve coursepeaks of can the be studied observed silverskin, with a maximum two main exothermic at 265 and 340peaks◦C. can It can be observed be concluded with a that maximum they represent at 265 andthe above-mentioned340 °C. It can be components,concluded that namely they hemicelluloserepresent the andabove cellulose.‐mentioned The obtainedcomponents, results namely are in hemicelluloseagreement with and the cellulose. experimental The data obtained presented results by Bry´setare in al.agreement [36] and Yang with et the al. [experimental37], who observed data a presentedsimilar tendency by Bryś inet the al. study [36] and of hemicellulose, Yang et al. [37], cellulose who observed and lignin a pyrolysis.similar tendency According in the to data study given of hemicellulose,in the literature, cellulose thermal and transitions lignin ofpyrolysis. hemicellulose According and cellulose to data are given present in atthe temperatures literature, thermal range of transitions220–315 and of 315–400hemicellulose◦C, with and maximum cellulose ofare peaks present found at temperatures at 268 and 355 range◦C, respectively. of 220–315 and According 315–400 to °C,the with results maximum obtained of by peaks Tarrio-Saavedra found at 268 etand al. 355 [38 ],°C, cellulose, respectively. lignin According and hemicellulose to the results decompose obtained at bytemperatures Tarrio‐Saavedra ranging et al. between [38], cellulose, 240–350, lignin 280–500 and hemicellulose and 200–260 decompose◦C, respectively. at temperatures Based on ranging the data betweenobtained 240–350, in this study 280–500 and and presented 200–260 by °C, other respectively. researchers, Based it can on be the concluded data obtained that hemicellulose in this study isand the presentedleast thermostable by other researchers, and lignin is it the can most be concluded thermostable that component hemicellulose in studied is the least material. thermostable and lignin is the most thermostable component in studied material. 3.1.2. TG/DTG Analysis of Coffee Silverskin 3.1.2. TG/DTGThe TG curve Analysis showing of Coffee the changesSilverskin in the mass of the silverskin sample during heating under oxygen until 700 C is presented in Figure2a. The TG curve◦ showing the changes in the mass of the silverskin sample during heating under The course of the TG/DTG curve shows that the thermal decomposition of the silverskin sample oxygen until 700 °C is presented in Figure 2a. occurs in three defined stages. In the case of the first event at the temperature range up to 150 ◦C, a mass loss of 3.6% was observed. Such a low weight-loss rate in the initial stage can be attributed to evaporation of moisture from inside the studied material. This event corroborates the data obtained from the DSC curve for the first event of thermal transition, confirming a possible dehydration of the material. It is worth mentioning that mass loss in this temperature range can be also related to releasing of volatile compounds present in coffee silverskin. The second and third mass losses of 64.9% are related to the thermal decomposition of organic matter present in the studied material. The most significant changes in weight occur in the region of 200–500 ◦C. At this temperature, decomposition of polysaccharides and some oils, which are the components of the sample, occurs. The final region of the Appl. Sci. 2020, 10, 8790 7 of 15

TG curve allows to determine the percentage of residues. Based on the course of TG/DTG, it can be seen that the silverskin sample is not fully decomposed in the studied range of temperatures. At the temperature of 700 ◦C, still 31.6% of the material remains undecomposed, which can be due to the presenceAppl. Sci. of 2020 inorganic, 10, x FOR compoundsPEER REVIEW with high stability. The course of the TG/DTG curve obtained7 of 16 in the case of analysis of silverskin conducted under nitrogen is shown in Figure2b.

Figure 2. Cont. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 16 Appl. Sci. 2020, 10, 8790 8 of 15

Figure 2. (a) Thermogravimetry/derivative thermogravimetry (TG/DTG) curves of coffee silverskin Figure 2. (a) Thermogravimetry/derivative thermogravimetry (TG/DTG) curves of coffee silverskin in oxygen atmosphere. (b) TG/DTG curves of coffee silverskin in nitrogen atmosphere. (c) TG/DTG in oxygen atmosphere. (b) TG/DTG curves of coffee silverskin in nitrogen atmosphere. (c) TG/DTG curves of coffee silverskin fat samples in oxygen atmosphere. (d) TG/DTG curves of coffee silverskin curves of coffee silverskin fat samples in oxygen atmosphere. (d) TG/DTG curves of coffee silverskin fat samples in nitrogen atmosphere. fat samples in nitrogen atmosphere. Similarly to the results obtained for experiments under oxygen, the first event in the temperature The course of the TG/DTG curve shows that the thermal decomposition of the silverskin sample range up to 175 ◦C, with the mass loss of 3.9%, was associated with the evaporation of moisture. occurs in three defined stages. In the case of the first event at the temperature range up to 150 °C, a The second and third mass losses of 42.1% and 7.0%, respectively, were observed in the temperature mass loss of 3.6% was observed. Such a low weight‐loss rate in the initial stage can be attributed to range from 200 to 500 ◦C. Based on the curve course, it can be observed that still 47.06% of the sample evaporation of moisture from inside the studied material. This event corroborates the data obtained remains undecomposed. from the DSC curve for the first event of thermal transition, confirming a possible dehydration of the material.3.1.3. Glass It is Transitionworth mentioning Temperature that of mass Coff eeloss Silverskin in this temperature Sample range can be also related to releasing of volatile compounds present in coffee silverskin. The second and third mass losses of In the analysis of silverskin as a by-product for potential further applications, it is of great 64.9% are related to the thermal decomposition of organic matter present in the studied material. The importance to determine the material’s stability. Taking the above into consideration, the glass most significant changes in weight occur in the region of 200–500 °C. At this temperature, transition (T ) temperature of the silverskin sample, which has been proven to be an effective indicator decompositiong of polysaccharides and some oils, which are the components of the sample, occurs. for food quality changes during storage, was determined by using MDSC method. It is worth The final region of the TG curve allows to determine the percentage of residues. Based on the course mentioning, that the most important change in the amorphous state occurs over the glass transition of TG/DTG, it can be seen that the silverskin sample is not fully decomposed in the studied range of temperature, which is the temperature at which polymeric materials change from an amorphous temperatures. At the temperature of 700 °C, still 31.6% of the material remains undecomposed, which solid (glass) to an amorphous rubber [39]. In the studied material, at water activity 0.419 (as it is), can be due to the presence of inorganic compounds with high stability. The course of the TG/DTG two glass transition events at 26.80 and 58.96 C were observed, which are associated with the curve obtained in the case of analysis− of silverskin conducted◦ under nitrogen is shown in Figure 2b. transitions of monosaccharides and disaccharides present in the sample, respectively. In the anhydrous Similarly to the results obtained for experiments under oxygen, the first event in the temperature state, most monosaccharides, such as pentoses: arabinose, ribose and xylose and hexoses, such as range up to 175 °C, with the mass loss of 3.9%, was associated with the evaporation of moisture. The rhamnose, fructose, galactose, glucose and mannose, are characterized by T temperature ranging second and third mass losses of 42.1% and 7.0%, respectively, were observed in theg temperature range from 20 to 31 C, and most of the disaccharides (sucrose, maltose, trehalose and lactose)—from 62 from 200− to 500 °C.◦ Based on the curve course, it can be observed that still 47.06% of the sample to 101 C. Analysis of the reported T values of anhydrous carbohydrates indicates that the higher remains undecomposed.◦ g the molecular weight of the component, the higher its Tg. Hydration of samples reduces their glass 3.1.3.transition Glass Transition temperature, Temperature i.e., the higher of Coffee the degree Silverskin of hydration Sample of the material is, the lower Tg value.

3.2.In Study the analysis of Coffee Silverskinof silverskin Oil as a by‐product for potential further applications, it is of great importance to determine the material’s stability. Taking the above into consideration, the glass transition3.2.1. TG (T/DTGg) temperature Analysis of Coof fftheee Silverskinsilverskin Oilsample, which has been proven to be an effective indicatorOne for of food the components quality changes of silverskin during storage, that is worth was determined being isolated by and using reused MDSC in othermethod. industrial It is worthfields mentioning, due to its potential that the most functional important and nutritionalchange in the properties amorphous is fat. state There occurs is no over information the glass in transitionthe literature temperature, dealing withwhich silverskin is the temperature oil characteristics. at which Taking polymeric the above materials into consideration, change from one an of amorphousthe goals ofsolid the (glass) study wasto an fat amorphous extraction rubber from the [39]. silverskin In the studied sample material, in order at to water define activity its properties 0.419 (asby it meansis), two of glass TG, transition PDSC and events GC techniques. at −26.80 and The 58.96 profiles °C were of the observed, TG and which DTG curvesare associated of fat samples with the transitions of monosaccharides and disaccharides present in the sample, respectively. In the anhydrous state, most monosaccharides, such as pentoses: arabinose, ribose and xylose and hexoses, Appl. Sci. 2020, 10, 8790 9 of 15 extracted from coffee silverskin, analyzed in oxygen and nitrogen atmospheres, are presented in Figure2c,d, respectively. The thermal decomposition of fat samples in oxygen occurs in three stages. The most significant changes in weight, associated with 60% of mass loss, are observed in the region of 150 to 350 ◦C and correspond to the decomposition of polyunsaturated fatty acids (PUFAs). It is thought that the initial step associated with unsaturated fatty acid decomposition is of great importance in defining the thermal stability of studied oils. The second step in the thermal decomposition of silverskin oil with 16% of mass loss is present in the temperature range from 350 to 450 ◦C and corresponds to the decomposition of monounsaturated fatty acids, such as oleic acid. The third event in the thermal decomposition, which occurs in a region of 450–560 ◦C, followed by 24% of mass loss corresponds to the thermal decomposition of saturated fatty acids, such as palmitic and stearic acids. It is worth mentioning that at the temperature of 560 ◦C, the decomposition of material is complete. These results are in agreement with study of Gouveia de Souza et al. [40], who determined thermal properties of sunflower oils with artificial antioxidants and without addition of artificial antioxidants. The authors observed three thermal steps in the range of 230 to 550 ◦C on TG/DTG curves, related to the decomposition of polyunsaturated, monounsaturated and saturated fatty acids, with no residue remaining at 800 ◦C. The thermal decomposition of fat samples in nitrogen atmosphere occurs in two stages ranged from 150 to 300 ◦C in the case of first event with 37% of loss mass to 300–520 ◦C for the second step, associated with 63% of loss mass.

3.2.2. PDSC Study of Coﬀee Silverskin Oil Induction time is essential in the terms of fat stability and can be used as a primary parameter for the assessment of the resistance of the tested fat to its thermal oxidative decomposition. The PDSC analysis of fats was performed at isothermal conditions. The DSC curves presenting the oxidation induction times are shown in Figure3. The time of oxidation reaction reached the values of 22, 48 and 108 min for experiments conducted at a temperature of 100, 110 and 120 ◦C, respectively. Comparing the obtained results with other oils, it can be stated that oil extracted from silverskin is of low stability. The oxidation induction time for rapeseed oil, hazelnut oil and extra virgin oil at 100 ◦C reached 200, 141 and 155 min, respectively [41,42], while in the case of silverskin oil—about 105 min. A similar Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 16 tendency was observed in the case of applying higher temperatures (110 and 120 ◦C).

FigureFigure 3. 3.DSC DSC curves curves of of oxidation oxidation induction induction time time of of silverskin silverskin fat fat samples. samples.

TheThe DSC DSC technique technique was was also also used used in in the the study study of of thermokinetic thermokinetic parameters parameters of of the the oxidation oxidation process. The results obtained show the eﬀect of the sample heating rate on the Ton and Tmax temperature process. The results obtained show the effect of the sample heating rate on the Ton and Tmax values.temperature With the values. increase With in the the increase heating in rate, the the heating temperature rate, the for temperature initiating thefor oxidationinitiating the process oxidation and process and the temperature for the maximum of the peak increased. The temperature of the extrapolated oxidation onset temperature (Ton) and the oxidation temperature corresponding to the maximum peak (Tmax) were recorded and are summarized in Table 1.

Table 1. Ton and Tmax obtained for different heating rates in thermo‐oxidation process of fat extracted from silverskin.

Heating Rate (β) Type of Fat Ton (°C) Tmax (°C) (K/min) 2.5 129.46 ± 2.8 206.68 ± 2.6 4.0 135.20 ± 3.2 213.55 ± 3.9 Fat extracted 5.0 138.70 ± 3.9 218.01 ± 4.4 from coffee silverskin 7.5 143.23 ± 2.4 229.72 ± 6.7 10.0 151.08 ± 1.9 237.94 ± 3.5 12.5 156.21 ± 3.1 241.21 ± 3.1

Based on the obtained extrapolated oxidation onset temperature (Ton) and the oxidation temperature corresponding to the maximum of the peak (Tmax) and considering the heating rate, graphs of the logarithm of heating rate versus temperature for oxidation of silverskin oil were prepared and are presented in Figures 4 and 5. Appl. Sci. 2020, 10, 8790 10 of 15 the temperature for the maximum of the peak increased. The temperature of the extrapolated oxidation onset temperature (Ton) and the oxidation temperature corresponding to the maximum peak (Tmax) were recorded and are summarized in Table1.

Table 1. Ton and Tmax obtained for diﬀerent heating rates in thermo-oxidation process of fat extracted from silverskin.

Type of Fat Heating Rate (β) (K/min) Ton (◦C) Tmax (◦C) 2.5 129.46 2.8 206.68 2.6 ± ± 4.0 135.20 3.2 213.55 3.9 ± ± Fat extracted 5.0 138.70 3.9 218.01 4.4 ± ± from coﬀee silverskin 7.5 143.23 2.4 229.72 6.7 ± ± 10.0 151.08 1.9 237.94 3.5 ± ± 12.5 156.21 3.1 241.21 3.1 ± ±

1.21.2 y =y − =4477.9x −4477.9x + 11.562 + 11.562 R² = 0.9773 1.01.0 R² = 0.9773

0.80.8 rate

rate

0.60.6 heating

heating 0.4 0.4log log 0.2 0.2

0.0 0.0 0.00232 0.00234 0.00236 0.00238 0.00240 0.00242 0.00244 0.00246 0.00248 0.00250 0.00232 0.00234 0.00236 0.00238 0.002401/Ton [1/K]0.00242 0.00244 0.00246 0.00248 0.00250 1/Ton [1/K] Figure 4. Temperature shift at the onset (of the oxidation process) of DSC curves depending on the FigureFigure 4.logarithmTemperature 4. Temperature of heating shift rate shift at of at thethe the thermo onset onset‐ (ofoxidative (of the the oxidationoxidation decomposition process) process) of silverskin of of DSC DSC curves fat. curves depending depending on the on the logarithmlogarithm of heating of heating rate rate of theof the thermo-oxidative thermo‐oxidative decompositiondecomposition of of silverskin silverskin fat. fat.

1.2 y = −4620.6x + 10.069 1.2 R² = 0.986 1.0 y = −4620.6x + 10.069 R² = 0.986 1.0 0.8 rate 0.8 0.6 rate

heating 0.6 0.4 log heating

0.4 log 0.2

0.2 0.0 0.00192 0.00194 0.00196 0.00198 0.00200 0.00202 0.00204 0.00206 0.00208 0.00210 1/Tmax [1/K] 0.0 0.00192 0.00194 0.00196 0.00198 0.00200 0.00202 0.00204 0.00206 0.00208 0.00210 Figure 5. Temperature shift at the maximum (of1/Tmax the oxidation [1/K] process) of DSC curves depending on the logarithm of heating rate of the thermo‐oxidative decomposition of silverskin fat. Figure 5. Temperature shift at the maximum (of the oxidation process) of DSC curves depending on Figure 5. Temperature shift at the maximum (of the oxidation process) of DSC curves depending on theThe logarithm dependencies of heating shown rate of in the Figures thermo 4 ‐andoxidative 5 can decompositionbe described by of a silverskin regression fat. equation: the logarithm of heating rate of the thermo-oxidative decomposition of silverskin fat. Log β = a(1/Ton or Tmax) + b (1) The dependencies shown in Figures 4 and 5 can be described by a regression equation: where β is a heating rate and T is the temperature. Log β = a(1/T ) + b Activation energy (Ea) values and pre‐exponentialon or factor Tmax (Z) were calculated using Equations (2)(1) whereand β (3),is respectively.a heating rate and T is the temperature. The activation energy values were calculated according to the Ozawa–Flynn–Wall method based Activation energy (Ea) values and pre‐exponential factor (Z) were calculated using Equations (2) on the equation: and (3), respectively. The activation energy values were calculated accordingd log  to the Ozawa–Flynn–Wall method based Ea = −2.19 R (2) on the equation: d1/T where R is the gas constant. d log  Ea = −2.19 R (2) The pre‐exponential factor was calculated from thed1 dependence:/T

where R is the gas constant. The pre‐exponential factor was calculated from the dependence: Appl. Sci. 2020, 10, 8790 11 of 15

The dependencies shown in Figures4 and5 can be described by a regression equation:

Log β = a(1/Ton or Tmax) + b (1) where β is a heating rate and T is the temperature. Activation energy (Ea) values and pre-exponential factor (Z) were calculated using Equations (2) and (3), respectively. The activation energy values were calculated according to the Ozawa–Flynn–Wall method based on the equation: d log β Ea = 2.19 R (2) − ·d(1/T) where R is the gas constant. The pre-exponential factor was calculated from the dependence:

Ea βEae RT Z = (3) RT2 Quantified regression coefficient values a, regression constant b and the determination coefficients R2, as well as values of activation energy and pre-exponential factor, are summarized in Table2.

Table 2. Thermokinetic parameters of oxidative process of coﬀee silverskin fat.

Parameter Value Calculated for Ton Value Calculated for Tmax a 4477.9 4620.6 − − b 11.6 10.1 R2 0.98 0.90 Ea (kJ/mol) 81.52 84.12 Z 7.68 109 2.39 108 × ×

In the present study, Ea reached the value of 81.52 kJ/mol (calculated for Ton) and 84.12 kJ/mol (calculated for Tmax). In terms of oils, the Ea value is aﬀected mainly by the degree of unsaturation. It was previously reported that a high polyunsaturated fatty acid (PUFA) content would decreased, while high monounsaturated fatty acid (MUFA) and saturated fatty acid (SFA) content would increase, the Ea value for the lipid oxidation process [43]. When compared with other oils—hazelnut oil (Ea = 89.1 kJ/mol) and rapeseed oil (Ea = 92.69 kJ/mol)—it can be stated that silverskin oil is of low resistance to oxidation. Taking the above into consideration, the research was completed by deﬁning fatty acids’ composition, which was carried out by gas chromatographic analysis of fatty acid methyl esters.

3.2.3. GC Study of Coffee Silverskin Oil The results obtained for the fatty acids composition of silverskin oil are reported in Table3. A high unsaturated fatty acid content (UFA) (50.38%) was indicated, with 7.81% monounsaturated fatty acids and 42.57% polyunsaturated fatty acids. Among the unsaturated fatty acids, linoleic acid (C18:2, 32.47%) as a main acid followed by oleic acid (C18:1, 7.08%) was detected. Additionally, the obtained results showed significant amounts of gamma-linolenic acid, belonging to the n-6 family, and alpha-linolenic acid, eicosatrienoic acid and eicosapentaenoic acid of the n-3 family. Based on the obtained results, it can be stated that silverskin oil is an abundant source of PUFAs. Among the saturated fatty acids, the predominant fatty acid was palmitic acid, which accounted for 28.26% of fatty acids. Next to palmitic acid, high stearic acid (8.04%) and arachidic acid (7.41%) contents were present in the triacylglycerol structure of silverskin oil. A typical chromatogram of fatty acids’ composition in the tested coffee silverskin fat is shown in Figure6. Appl. Sci. 2020, 10, 8790 12 of 15

Table 3. Fatty acids composition (%) of coﬀee silverskin fat samples (SFA—saturated fatty acids, MUFA—monounsaturated fatty acids, PUFA—polyunsaturated fatty acids).

Group of Fatty Acids Fatty Acid Content of Individual Fatty Acid (%) * Σ Content of the Group (%) * C12:0 0.08 0.01 ± C14:0 1.86 0.06 ± C15:0 0.19 0.01 ± C16:0 28.26 0.09 SFA ± 46.37 0.05 C18:0 8.04 0.05 ± ± C20:0 7.41 0.09 ± C21:0 0.33 0.04 ± C24:0 0.20 0.04 ± C18:2n-6c 32.47 0.09 ± C18:3n-6 1.62 0.07 ± C18:3n-3 1.44 0.04 ± PUFA C20:2 0.10 0.02 42.58 0.25 ± ± C20:3 n-3 5.50 0.06 ± C22:2 0.23 0.04 ± C20:5 n-3 1.22 0.02 ± C16:1 0.10 0.01 ± C18:1n-9c 7.08 0.04 MUFA ± 7.81 0.05 C20:1 c 0.51 0.02 ± ± C24:1 0.12 0.02 ± Σ identiﬁed 96.76 0.20 ± Σ other minor unidentiﬁed 3.24 0.20 ± * Data expressed as mean standard deviation. ±

Figure 6. Typical chromatogram of fatty acids proﬁle in coﬀee silverskin fat (1—C12:0; 2—C14:0; 3—C15:0; 4—C16:0; 5—C16:1; 6—C18:0; 7—C18:1; 8—C18:2; 9—C18:3 n-6; 10—C18:3 n-3; 11—C20:0; 12—C20:1; 13—C21:0; 14—C20:2; 15—C20:3; 16—C22:2; 17—C24:0; 18—C20:5; 19—C24:1).

Additionally, in the case of silverskin oil, the n-6 to n-3 fatty acids ratio was calculated. The parameter should be considered in the fat analysis as it is of high importance in the characteristics of fat from a nutritional point of view. Nowadays, the consumption of n-6 fatty acids in the diet is at a high level, which leads to an unhealthy n-6 to n-3 ratio of 20:1 or even higher, which is associated with the risk of heart disease and brain health consequences. It is worth mentioning that commonly used oils, such as sunﬂower, corn, soybean and cottonseed oils, contain a high amount of n-6-family fatty acids. In the present study, for silverskin oil, the n-6 to n-3 ratio value reached 4:1, which is in

Appl. Sci. 2020, 10, 8790 13 of 15 agreement with recommendations and meets the trend of searching for new sources of fats which provide a beneﬁcial balance of n-6 to n-3 fatty acids.

4. Conclusions The obtained results contributed to better characterization of coffee silverskin as a valuable by-product of the coffee roasting process. Considering the TG/DTG curves, a well-defined step of silverskin decomposition can be observed. The most significant mass losses of 64.9% occurred in the region of –500 ◦C and are related to the thermal decomposition of organic matter present in the studied material. Undecomposed amounts of the sample can be related to the significant content of inorganic residue with high stability. The course of the DSC curve confirmed the observed phenomena. The DSC profile indicates the occurrence of two main exothermic peaks, which represent the main components of silverskin, hemicellulose and cellulose. In the case of isolated fat, the most significant changes in weight, associated with 60% of mass loss, correspond to the decomposition of polyunsaturated fatty acids. It has been proven that silverskin is a rich source of PUFAs, with linoleic acid as a main fatty acid followed by oleic acid. Gamma-linolenic, alpha-linolenic, eicosatrienoic and eicosapentaenoic acids were also detected in the structure of triacylglycerols. The second and third steps in the thermal decomposition of silverskin oil with 16% and 24% mass loss were associated with decomposition of monounsaturated fatty acids and saturated fatty acids, respectively. The presence of these two groups of fatty acids was confirmed by means of gas chromatography. Fat extracted from the studied coffee by-product proved to be a valuable source of essential fatty acids with the recommended n-6 to n-3 ratio and could be successfully recovered and used in other fields of industry, including, especially, the food industry, which fits perfectly with current trends in circular ecology. Based on the obtained results, it can be stated that fat isolated from silverskin can be treated as a value-added bioactive material that can be obtained from coffee roasting by-product. To the best of our knowledge, this is the first study on silverskin fat analysis.

Author Contributions: Conceptualization, A.G. and R.B.; methodology, A.G., E.O.-L., E.D. and J.B.; software, A.G. and R.B.; validation, A.G., E.O.-L., M.W.-W. and J.B.; formal analysis, A.G., E.O.-L. and R.B.; investigation, A.G., E.O.-L. and R.B.; data curation, A.G. and R.B.; writing—original draft preparation, A.G. and R.B.; writing—review and editing, A.G.; visualization, A.G. and R.B.; supervision, A.G. and R.B.; project administration, A.G. and R.B.; funding acquisition, A.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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